1. Field of the Invention
The present invention generally relates to image processing methods, image processing apparatuses, programs, and recording media that embed digital watermarks in digital content by encoding, and detect copyright information and/or falsification in the process of decoding the digital watermarks embedded in the digital content.
2. Description of the Related Art
Digital information can be easily copied by, for example, computers, without being degraded. Conversely, digital information may be easily overwritten and/or falsified. Hence, with simple processes and/or operations, digital information is likely to be illegally copied and reused without permission, or may be partially altered such that the digital information cannot be used as evidence photography.
Methods for preventing such problems include a method called digital watermarking or data hiding. Digital watermarking is a method of adding information that is invisible when digital contents such as digital images are normally reproduced.
Methods for embedding digital watermarks may be broadly classified into the following two categories:
(1) Method for directly embedding into a sample value of content data
(2) Method for embedding into a frequency component.
In the method (1), when processes such as compression are performed, embedded data are likely to be lost, but the processing workload is light. On the other hand, in the method (2), embedded data are not easily lost in processes such as compression, but the processing workloads of embedding and extracting are heavy.
It is difficult to apply the method (2) to digital watermarking in the cases where data are saved in data formats that are difficult to be expressed in gradation, such as binary images and color images that only allow use of specified colors. Thus, the method (1) is generally used for digital watermarking in such cases. As described below, the present invention is realized with the method (1).
Intended purposes of adding digital watermarks to digital contents include, for example: recording of copyright information; tracing of information of illegal copiers; recording of IP address history; prevention of illegal copying (invisible/high tolerance type); application to prevention of falsification (invisible/low tolerance type); authentication; secret communications; embedding of remarks and/or labels of digital contents (visible/invisible type, displaying owner); allowing for removal of watermarks (visible/invisible type, content distribution).
A method has been proposed to provide a camera with a mechanism for embedding the serial number of the camera and shooting date and time, and simultaneously creating an electronic signature when taking pictures (refer to “Research Report on Digital Watermark Techniques”, Japan Electronic Industry Development Association, March, 1993, and ‘Prevention of Illegal copying of Electronic Information’, pp. 68-70 of “Nikkei Business”, Feb. 23, 1998). Thereby, in addition to detection of falsification of evidence photography, it is possible to specify the camera used for taking pictures and confirm the date and time when the pictures were taken. Falsification may be detected only by electronic signature techniques. However, with the use of watermarking techniques, it is possible to specify the camera used for taking pictures and the date and time when the pictures were taken, which is more effective for preventing falsification. Additionally, by increasing the amount of information within embedded digital watermarks, the accuracy in specifying the falsified position is improved. Further, in terms of copyright protection, the more the embedded information of watermarks is, the easier the extraction of the information becomes when analyzing the embedded information, which contributes to the effectiveness.
However, embedding a digital watermark degrades digital content data in which the digital watermark is to be embedded. Thus, it is important to degrade image quality as little as possible even if the amount of embedded data is increased.
Meanwhile, in the case where a digital watermark is directly embedded in an image by taking advantage of the characteristic “human eyes detect noise in edge portions of the image less than noise in flat portions of the image”, the digital watermark is generally embedded in the edge portions of the image. However, normally, in the image, the area of the edge portions is less than that of the flat portions. Thus, when the amount of information of a digital watermark to be embedded is large, the image quality is significantly degraded. Particularly, the information and/or meaning of an original image may be lost when degradation of the image quality is such that the outline of the image (e.g., ledger sheet, figure, or map) that is artificially created is excessively affected, or such that the framework of the image is divided or deleted.
That is, in order to improve performance in detecting falsification and/or extracting copyright information, it is preferable to increase the amount of information of an embedded watermark as much as possible while degrading the image quality as little as possible. However, the more the amount of information of an embedded watermark becomes, the higher the level of degradation of image quality becomes. In other words, there is a trade-off relationship between the amount of information of an embedded watermark and the level of degradation of image quality.